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Interpretive Summary: Phytophthora species are among plant pathogens that are the most threatening to agriculture. Some threatening species have not been isolated in the U.S. yet. One important avenue for the importation of these species is on ornamental plants. Currently there are 17 species that are known to be pathogenic on Rhododendrons. Based upon stable characteristics that can be easily identified in culture, a dichotomous key was produced that differentiates these species from one another. This key will benefit diagnostic labs, extension agents, and regulatory agents to identify potentially new imported species quickly without the use of molecular methods or an extensive culture collection.

Technical Abstract: Phytophthora species are among plant pathogens that are the most threatening to agriculture. After the discovery of P. ramorum, surveys have identified new species and new reports on Rhododendrons. Based upon propagule production and characteristics and colony growth, a dichotomous key was produced that can differentiate the majority of the 17 Phytophthora sp. known to be pathogenic to aerial plant parts of Rhododendrons. These distinctions were made without molecular tools and wide-ranging variables such as propagule sizes and can be made without the need for a large culture collection.